Natural gas is becoming more important as the world's energy demand increases. Natural gas is readily available, in particular with the new technologies now employed to extract and utilize shale gas. Natural gas is much cleaner burning than oil and coal, and does not have the hazard or waste deposition problems associated with nuclear power. The post-combustion emission of greenhouse gases from natural gas is lower than for oil, and only about one third of such emissions from coal.
There is substantial international trade in natural gas, and its price varies significantly in different parts of the world. A large fraction of this trade is in the form of liquefied natural gas (LNG). LNG is produced using two major processing steps. The first step is gas pre-treatment to remove components that can solidify when cooled to cryogenic temperatures, mainly sour components and water. Trace elements, mainly mercury that can form amalgams—in particular with aluminium process components—are also removed from the gas.
Heavy hydrocarbon fractions or Natural Gas Liquids (NGL) may be removed from the gas in the first or second of the two LNG processing steps. The second processing step is mainly liquefaction of the purified gas, composed mainly of methane. This methane, with small amounts of heavier components, is liquid at atmospheric pressure and about −163° C. After liquefaction, the LNG is shipped to the destination and re-gasified.
Processing of natural gas to produce LNG has traditionally been done at large land-based facilities, which include the two steps of pre-treatment and liquefaction at the same location. Recent developments in technology and markets have enabled construction of LNG plants on floating structures, a development that has inspired movement of a substantial portion of LNG processing facilities offshore to Floating Liquefied Natural Gas facilities (FLNG).
Floating LNG production units are also known. WO2010036121 describes a method and system for handling gas, where an liquefaction unit is arranged onboard a LNG carrier for liquefaction of natural gas to be transported in tanks onboard the carrier. Arranging the LNG liquefaction unit onboard the carrier allows for transporting natural gas as LNG from terminals lacking LNG liquefaction possibilities. The LNG liquefaction unit described therein is a standard LNG unit where a cooling unit for cooling nitrogen as cooling medium for the liquefaction is cooled by water coolers.
WO2010059059 describes a device for floating production of LNG, comprising a LNG carrier comprising at least one LNG tank, where projecting hull structures are fixed to the ship hull and are used for arranging the LNG liquefaction unit onboard. Nothing is mentioned therein about the LNG liquefaction unit as such. WO2013156623 relates to a LNG plant comprising a first and a second converted LNG carrier, connected to form a catamaran, and where at least one LNG tank has been removed to create a space for the LNG liquefaction unit. Nothing is mentioned about the LNG liquefaction unit as such.
The FLNGs are typically designed to be located at a distance from a coast and are connected to natural gas supplies by pipelines. The FLNGs typically are also designed to serve as buffer LNG storage and as terminals for loading of LNG tankers that are used for transport of the LNG to the markets.
The recent development towards FLNGs has made offshore natural gas resources more available to the market, and has resulted in a reduction of capital cost for establishing a LNG plant. Other key drivers for offshore liquefaction include reduction of onshore environmental impacts; reduction of land use issues for equipment and infrastructure; and reduced likelihood of opposition from local communities. An entire FLNG plant can be built in a shipyard, which is efficient and improves quality control, cost control and reduces construction time. FLNG's are also mobile and can be transferred to alternative locations if required.
Numerous studies of FLNG technologies have been carried out over the last couple of decades. Currently, several projects are underway worldwide. To date, actual construction has started for three units: the Shell Prelude project, the Exmar/Pacific Rubicales barge project, and the Petronas FLNG 1 project.
In these and other projects, both gas pre-processing and liquefaction will typically be located on the deck of the FLNG. Space below deck is used for LNG storage and marine-specific equipment. The area available on an FLNG deck is generally less than 20% of the area used for similar facilities onshore. Among other design constraints, this reduced process layout space presents safety issues, including proximity to living quarters and limited space for safety barriers. Significantly, it also limits the size of the processing plants and the possibilities to utilize economies of scale.
Accordingly, in addition to safety issues, the liquefaction process involves environmental issues. The liquefaction process generates large amounts of heat, which must be transferred to the environment. With current designs, large amounts of seawater are needed for cooling purposes onboard the FLNG, water that is subsequently discharged back to the sea at a higher temperature. This can be harmful to marine life because of mechanical stress in sea water pipes, pumps and fittings, use of toxic chemicals to prevent fouling, and the increased water temperature. In coastal waters, where marine life is abundant, certain jurisdictions do not allow the use of seawater for cooling at all, with others expected to follow.
The alternative to seawater cooling is air-cooling. However, air-cooling requires substantially more space than seawater cooling. This space is proportional to the cooling duty and to the Logarithmic Mean Temperature Difference (LMTD) between the air and the process fluid to be cooled. The footprint of a well-designed air cooling system, typically with an LMTD of 30° C., might be in the order of 1000 m2 per 100 MW cooling duty. This is a challenge, in particular when available space is less than 20% of the space used on-shore for similar plants.
Liquefaction plants are typically either efficient base-load systems, or less efficient but simpler peak-shaving systems. Known refrigerants, such as hydrocarbons or nitrogen, circulate in cooling systems comprising compressors, air-cooled heat exchangers, and LNG exchangers. Depending on the refrigeration system, refrigerants may or may not be condensed in the air coolers before being routed to the LNG exchangers.
In normal situations, it would be desirable or required to cool/condense the refrigerants to about 30° to 40° C. before the refrigerants are routed to the LNG exchangers. However, in temperate areas, design ambient air temperature may be relatively high (such as 32° C. (90° F.)) or higher, and it is anticipated that the approach temperature for the air cooled heat exchangers should be at least 10° C., preferably 15° C. or more.
Engineers skilled in the art will know that this problem can be solved by operating compressor inter-stage coolers at higher temperatures, and compressing the refrigerants, especially any refrigerant which shall condense, to higher pressure than normal. All cooling and condensation therefore takes place at higher temperatures, enabling air-cooling with high LMTD and high air cooler approach temperatures even in temperate areas. Unfortunately, all of this significantly reduces the liquefaction efficiency, increases the energy demand and therefore increases the cooling duty, which partly defeats strategy of increasing the LMTD and accomplishing air-cooling.
Table 1, see below, illustrates this by comparing work and cooling duty for two liquefaction processes with water and air cooling. Liquefaction rate is 400 metric tons per hour, the feed gas is at 60 bara and 25° C., and consists of 98 mole % methane, 1.5 mole % ethane and 0.5 mole % propane.
TABLE 1Comparison of work and cooling duty for two liquefaction processesWater cooledAir cooledComprEnthalpyCoolingComprEnthalpyCoolingLiquefactionEfficiencydutychangedutyEfficiencydutychangedutysystem(kWh/kg)(MW)(MW)(MW)(kWh/kg)(MW)(MW)(MW)Base load0.312092.9212.90.3614492.9236.9Peak0.520092.9292.90.624092.9332.9shaving
As is evident from table 1, the cooling duty for air-cooled systems is substantially higher than the cooling duty for water-cooled systems.
Air-cooling as such is well known technology, widely used on-shore for power plants, buildings and many other purposes. FIG. 1 illustrates an air cooler 1 arranged on a rack 2. Incoming air for cooling is flowing into the air cooler from below as indicated by arrows 3, and outgoing air, and heated, air is released at the top of the cooler as indicated by arrows 4. Heat transfer medium circulation is indicated with lines 5. Air is drawn in from below, using a fan. This air passes over a coil, which contains the process fluid to be cooled. The air exiting from the air cooler is normally warmer than the ambient air, and will tend to rise because of the lower density. However, a part of the outgoing heated air may flow back into the air inlet and thus reduce the cooling efficiency.
Examples on air cooler arrangements for cooling of liquids and gases may be found e.g. in GB 903 397, describing arrangements of air coolers in parallel rows and where fans are disposed below the air cooler units to blow air from below and upwards through the air cooler units to avoid air heated by the air coolers to be recycled back through the air coolers and thereby reduce the cooling efficiency.
On a floater, with different wind directions and large arrays of air coolers, such recirculation may be likely, and would be detrimental to the performance of an air cooling system. On shore, this problem is partially solved by spreading the air coolers over a large area, and by providing a high air velocity out of the coolers.
Air coolers are also susceptible to fouling or deposition of contaminants on heat exchanging surfaces such as pipes or finned pipes containing the heat transfer medium. On floaters, such contaminants might be salt, smoke or oil mists. This reduces the heat transfer efficiency. In many situations, such fouling is predicted in advance, and the coolers are oversized accordingly. With the limited space on a floater, such oversizing may not be practical. It is known that gas turbines in coastal or offshore areas have similar problems. In this case, the solution has been to provide inlet air filters.
An additional challenge is determination of the design air temperature. In many areas, the annual average air temperature is much lower than the peak summer temperature. Furthermore, the peak summer temperature may occur only a few days per year. It might be highly desirable to size the air coolers based on an average temperature instead of the peak temperature, since this can significantly reduce the number of air coolers.
However, on warm days, this means that the cooling capacity may be severely reduced. In some cases, this problem is solved by using a design air temperature, which is lower than the peak annual temperature, and using a water spray at the air cooler inlet on very warm days. This reduces the air temperature to acceptable levels, because the air wet bulb temperature is usually lower than the dry bulb temperature.
The most important factor determining the economics of an FLNG is the LNG production rate. Higher production rate requires proportionally more cooling and increases the air cooler footprint correspondingly.
All of this shows that it is highly desirable to have as much air cooling capacity as possible, in particular on FLNGs where the space is limited.
A novel adaptation of FLNG, which increases the available space on a floater, is the Coastal Liquefaction, Storage and Offloading (CLSO) facility. The CLSO adaptation addresses FLNG space limitations, safety, environmental impact and the all-important processing capacity. The first processing step, gas pre-processing, is mainly performed on shore, on separate terminals or on dedicated floating systems, instead of occupying valuable space on the FLNG. Pre-processed gas is then piped to one or more floating CLSO's, which now have much more deck space available. Extra deck space on the CLSO, freed up by removing pre-processing, can be used for additional safety features. The extra deck space also opens the possibility of using air-cooling instead of seawater cooling, solving the seawater intake and associated environmental issue. However, it would be far better to use this space for extra production capacity, without falling back on seawater cooling, if possible. Furthermore, possibilities exist for greater liquefaction capacity and the resulting economic advantages.
A large CLSO might e.g. have a length of 350 m and width of 60 m, corresponding to existing hull dimensions for existing, successful vessel designs. The deck space is therefore about 20,000 m2. Desired production might be in the order of 1,000 metric tons LNG per hour for this size CLSO. According to Table 1, for an air-cooled base load system, this would require at least 600 MW of cooling. With air coolers requiring about 1,000 m2 per 100 MW cooling, this will require 6,000 m2 deck area or about one third of the CLSO deck.
Given those requirements, air coolers placed on deck introduce certain design and economic difficulties: it would be very difficult to ensure that all coolers get fresh air and not air that is partly recirculated and therefore too warm for effective operation; for such a large array of coolers, provision of inlet air filters and water spray on hot days would become unwieldy; and, since LNG production capacity depends on the available space air coolers, if placed directly on deck, would significantly reduce the space available for liquefaction capacity.
One design alternative is to locate the air coolers 1 on a cantilever 6 arranged at one side of the hull 7 as shown in FIG. 2. For a floater of the size mentioned above, the cantilever has to be about 17 m wide in the full length of the floater, e.g. about 350 m, to provide an area of 6,000 m2. In addition to this, areas for air cooler access ways may be required for maintenance. This is not very practical. In addition, the air coolers would be exposed to salty seawater mist. The need to provide filters and fresh water spray onto the coolers on hot days would further complicate this design approach.
Alternative configurations are shown in FIGS. 3 and 4. These configurations raise other issues such as height, hot air blown into the deck of the floater or non-symmetrical momentum created by the airflow from the coolers. Air recirculation, which is detrimental to the air cooler efficiency, would be a problem in all of these cases.
An object of the present invention is therefor to provide improvements in air cooler efficiency for such floaters as described above.